This invention relates to improvement in a metal vapor discharge lamp which is constituted in such a manner that a starter connected in parallel to a luminous tube is accommodated in a light transmissible outer bulb, and more particularly to improvement in a high pressure sodium lamp having a starter therein.
FIG. 1 shows an example of the structure of a circuit for use in a conventional metal vapor discharge lamp having a starter therein. A metal vapor discharge lamp of the type described above is constituted in such a manner that a starter 5 formed by a series circuit comprising a bimetal switch 2, a heater 3 and a limiting current resistor 4 is, in parallel, connected to a luminous tube 1, and the thus connected components are accommodated in a light transmissible outer bulb 6. When an A.C. voltage is applied to the thus-constituted discharge lamp which has been connected to an A.C. power source 8 via an inductor 7, an electric current passes through the starter 5 formed by a series circuit comprising a bimetal switch 2, a heater 3 and a limiting current resistor 4. As a result of this, the bimetal switch 2 repeats switching on/off operation. This leads to a fact that high voltage pulses are generated in the inductor 7, and are added, with the power source voltage, to the luminous tube 1, causing the discharge lamp to be lit.
By using the above-described starter which utilizes switching on/off operation performed by the bimetal switch 2, a substantially several thousand volts of high voltage pulses can be generated. Therefore, such discharge lamp can be started lighting by using a relatively small size ballast. However, since the mechanical switching on/off operation of the bimetal switch 2 is utilized, the heights and the intervals of the generated pulses cannot be uniformed, the starting characteristics of the discharge lamp is not stable, or an exceeding high voltage pulses are involved to be generated, causing the wiring instruments or the like to be damaged.
In order to overcome the above-described problems, a metal vapor discharge lamp was disclosed in which a nonlinear capacitor 9 thereof is, as shown in FIG. 2, connected, in parallel, to the luminous tube 1 with a thermal response type switch 10, and the thus-connected components are accommodated in a light-transmissible vacuum outer bulb 6. The starter of this discharge lamp utilizes the nonlinear capacitor 9 as a switching element depending upon the hysteresis characteristics of the charge of the nonlinear capacitor 9 with respect to the voltage thereof. The thermal response type switch 10 is provided for the purpose of separating the nonlinear capacitor 9 which serves as a starter from the luminous tube 1 after the discharge lamp has started lighting.
In this starter, since high voltage pulses are generated in the inductor 7 utilizing the electrical switching on/off operation of the nonlinear capacitor 9, the heights and intervals of the generated high voltage pulses are made very regular and stable. However, the thus-generated high voltage pulses are within the range of substantially 1000 to 3000V, this level being slightly lower than that in a case of a starter comprising a bimetal switch. Therefore, in order to assuredly start the discharge lamp, the nonlinear capacitor and its circuit elements need to be designed in such a manner that the high voltage pulses which are very close to the upper limit of the above-described pulses can be generated.
However, a fact was found that the starter and the like designed as described above will easily cause the following failures: when the light-emitting life of the discharge lamp reaches its final stage, causing the rare gases in the luminous tube 1 to be leaked in the vacuum outer bulb 6 or causing the vacuum outer bulb 6 to generate slow leak, the withstand voltage of the nonlinear capacitor 9 is reduced, as a result of which discharge occurs between its two electrodes. It leads to a fact that a large electric current is caused to be passed through the starter circuit. Consequently failures that the circuit elements such as the inductor 7 are damaged easily occur. Although the nonlinear capacitor 9 is finally broken due to discharge between the two electrodes of the nonlinear capacitor 9, causing the circuit of the starter to be shut off, a large current which passes through in a short time until the breakage occurs will cause the above-described failure.